Enhanced hard bias in thin film magnetoresistive sensors with perpendicular easy axis growth of hard bias and strong shield-hard bias coupling

ABSTRACT

A hard bias (HB) structure for longitudinally biasing a free layer in a MR sensor is disclosed that is based on HB easy axis growth perpendicular to an underlying seed layer which is formed above a substrate and along two sidewalls of the sensor. In one embodiment, a conformal soft magnetic layer that may be a top shield contacts the HB layer to provide direct exchange coupling that compensates HB surface charges. Optionally, a thin capping layer on the HB layer enables magneto-static shield-HB coupling. After HB initialization, HB regions along the sensor sidewalls have magnetizations that are perpendicular to the sidewalls as a result of surface charges near the seed layer. Sidewalls may be extended into the substrate (bottom shield) to give enhanced protection against side reading. The top surface of the seed layer may be amorphous or crystalline to promote HB easy axis perpendicular growth.

FIELD OF THE INVENTION

The invention relates to an improved hard bias (HB) structure havingperpendicular easy axis growth on a seed layer and stabilization fromshield-HB coupling which can increase the hard bias field strength andlower the hard bias field variation in thin film magneto-resistive (MR)sensors.

BACKGROUND OF THE INVENTION

In a magnetic recording device in which a read head is based on a spinvalve magnetoresistance (SVMR) or a giant magnetoresistance (GMR)effect, there is a constant drive to increase recording density. Onemethod of accomplishing this objective is to decrease the size of thesensor element in the read head that is suspended over a magnetic diskon an air bearing surface (ABS). The sensor is a critical component inwhich different magnetic states are detected by passing a sense currentthrough the sensor and monitoring a resistance change. A popular GMRconfiguration includes two ferromagnetic layers which are separated by anon-magnetic conductive layer in the sensor stack. One of theferromagnetic layers is a pinned layer wherein the magnetizationdirection is fixed by exchange coupling with an adjacentanti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layeris a free layer wherein the magnetization vector can rotate in responseto external magnetic fields. In the absence of an external magneticfield, the magnetization direction of the free layer is alignedperpendicular to that of the pinned layer by the influence of hard biaslayers on opposite sides of the sensor stack. When an external magneticfield is applied by passing the sensor over a recording medium on theABS, the free layer magnetic moment may rotate to a direction which isparallel to that of the pinned layer. Alternatively, in a tunnelingmagnetoresistive (TMR) sensor, the two ferromagnetic layers areseparated by a thin non-magnetic dielectric layer.

A sense current is used to detect a resistance value which is lower whenthe magnetic moments of the free layer and pinned layer are in aparallel state. In a CPP configuration, a sense current is passedthrough the sensor in a direction perpendicular to the layers in thesensor stack. Alternatively, there is a current-in-plane (CIP)configuration where the sense current passes through the sensor in adirection parallel to the planes of the layers in the sensor stack.

Ultra-high density (over 100 Gb/in²) recording requires a highlysensitive read head in which the cross-sectional area of the sensor istypically smaller than 0.1×0.1 microns at the ABS plane. Currentrecording head applications are typically based on an abutting junctionconfiguration in which a hard bias layer is formed adjacent to each sideof a free layer in a GMR spin valve structure. As the recording densityfurther increases and track width decreases, the junction edge stabilitybecomes more important so that edge demagnification in the free layerneeds to be reduced. In other words, horizontal (longitudinal) biasingis necessary so that a single domain magnetization state in the freelayer will be stable against all reasonable perturbations while thesensor maintains relatively high signal sensitivity.

In longitudinal biasing read head design, films of high coercivitymaterial are abutted against the edges of the sensor and particularlyagainst the sides of the free layer. By arranging for the flux flow inthe free layer to be equal to the flux flow in the adjoining hard biaslayer, the demagnetizing field at the junction edges of theaforementioned layers vanishes because of the absence of magnetic polesat the junction. As the critical dimensions for sensor elements becomesmaller with higher recording density requirements but sensor layerthickness decreases at a slower rate, the minimum longitudinal biasfield necessary for free layer domain stabilization increases.

A high coercivity in the in-plane direction is needed in the hard biaslayer to provide a stable longitudinal bias that maintains a singledomain state in the free layer and thereby avoids undesirable Barkhausennoise. This condition is realized when there is a sufficient in-planeremnant magnetization (M_(r)) from the hard bias layer which may also beexpressed as M_(r)t since hard bias field is also dependent on thethickness (t) of the hard bias layer. M_(r)t is the component thatprovides the longitudinal bias flux to the free layer and must be highenough to assure a single magnetic domain in the free layer but not sohigh as to prevent the magnetic field in the free layer from rotatingunder the influence of a reasonably sized external magnetic field.Moreover, a high squareness (S) hard bias material is desired. In otherwords, S=M_(r)/M_(S) should approach 1 where M_(S) represents themagnetic saturation value of the hard bias material.

Referring to FIG. 1, a generic TMR or CPP-GMR read head structure 40 isshown that is similar to read heads currently being employed inmanufacturing or in development. Read heads of this type are describedin the following references: R. Fontana and S. Parkin, “Magnetic tunneljunction device with longitudinal biasing”, U.S. Pat. No. 5,729,410; S.Mao et al., “Commercial TMR heads for hard disk drives: characterizationand extendibility at 300 Gbit/in²”, IEEE Trans. Magn., Vol. 42, No. 2,p. 97 (2006); and T. Kagami et al., “A Performance Study of NextGeneration's TMR Heads Beyond 200 Gb/in²”, IEEE Trans. Magn., Vol. 42,No. 2, p 93 (2006). The read head 40 is comprised of a bottom shield 1and a top shield 16 that also function as bottom and top electricalleads for conducting current through the sensor stack. Layers 6 through10 represent the sensor stack that is formed on the bottom shield 1 andis patterned by a well known method to form nearly vertical or slightlysloping sidewalls such that layer 6 has a larger length along the x-axisthan layer 10. Layer 6 is generally a multilayer structure that may havea seed layer, an anti-ferromagnetic (AFM) layer, and a pinned layer (notshown), being exchange coupled to the AFM layer, deposited sequentiallyon the bottom shield 1. A reference or second pinned layer 7 is on thelayer 6 and may have a synthetic anti-ferromagnetic (SAF) compositionlike the first pinned layer. The spacer layer 8 is usually an insulatorcomprised of a metallic oxide for TMR heads, or a metallic layer or alayer with metallic nano-channels for CPP GMR heads. Above the spacerlayer 8 is a free layer 9 and a layer 10 that may be a capping layer,for example.

An insulating layer 3 is formed along the sidewalls of the sensor stackand on the bottom shield 1 adjacent to the sensor stack and is typicallya metallic oxide that prevents shorting between the top and bottomshields as well as the sensor stack. A seed layer 4 disposed on theinsulating layer 3 is commonly used to promote the in-plane easy axis ofthe hard bias material during deposition of the hard bias layer 5 asmentioned in the following references: D. Larson et al. in U.S. Pat. No.7,061,731; P. Chau et al. in Publication No. US 2005/0066514; H. Gill inPublication No. US 2006/0114622; M. Pinarbasi in Publication No. US2006/0087772; and K. Zhang et al. in Publication No. US 2006/0132989.The seed layer 4 leads to the hard axis of the hard bias (HB) layer 5being grown perpendicular to the seed layer plane. Above the HB layer 5there is generally a capping layer 11 that has multiple purposes such asincreasing the spacing between the hard bias layer 5 and top shield 16,increasing the HB field uniformity, and reducing HB internal stressafter deposition.

In an ideal situation, the magnetization of the hard bias layer 5 isaligned longitudinally (along the x-axis) and parallel to the sensorfilm plane after the sensor is exposed to a strong magnetic fieldapplied in the direction of the arrows 12. This process is called thehard bias initialization step. Ideally, the magnetic charges 13 a, 13 b,created by the HB magnetization on the side edges of the sensor stackwill provide a longitudinal field in the free layer as a bias field.This field keeps free layer 9 magnetization longitudinal when noexternal field is applied. In the example shown in FIG. 1, the referencelayer 7 has a fixed magnetization along the y-axis as a result of anannealing process and coupling with the first pinned layer and AFM layerin layer 6. When a magnetic field of sufficient strength is applied inthe y direction from a recording medium by moving the read head 40 overa hard disk surface (not shown) oriented in the z direction, then themagnetization in the free layer 9 switches to the y directions. Thischange in magnetic state is sensed by a voltage change due to a drop orincrease in the electrical resistance for an electrical current that ispassed through the sensor. In a TMR or CPP spin valve, the sense currentbetween the top shield 16 and bottom shield 1 is in a directionperpendicular to the planes of the sensor stack.

Unfortunately, the ideal case of forming magnetic charges adjacent tothe sensor stack as depicted in FIG. 1 is not representative ofconditions in an actual read head since the read head 40 has someintrinsic properties that tend to cause significant HB field degradationand large HB field variations. These problems are especially significantin narrow shield-shield read head applications where the distancebetween the top and bottom shields is reduced in order to achieve highareal density magnetic recording.

Referring to FIG. 2 a, the growth pattern of the HB material in HB layer5 is shown for the longitudinal HB scheme described in FIG. 1. Only theleft portion of the sensor structure is shown in the drawing. Note thatthe insulating layer 3 has a section 3 a that is formed on the bottomshield 1 and a section 3 b along the side of the sensor stack. Since theseed layer 4 conforms to insulating layer 3 and promotes an in-planeeasy axis orientation, the easy axis of the HB layer 5 formed on theseed layer above section 3 a is parallel to the sensor film plane whilethe easy axis of the HB layer deposited on the seed layer on slopedsection 3 b will generally follow the slope direction. Arrows 14 a and14 b illustrate the possible growth pattern of the HB easy axis abovesections 3 a and 3 b, respectively. Area 15 where the two different easyaxis orientations meet is where a high stress or amorphous phase of theHB layer 5 will likely occur.

After HB initialization, HB magnetization will fall back on to the easyaxis directions shown in FIG. 2 a since the HB material usually has astrong uniaxial anisotropy along the easy axis. Therefore, the region ofthe HB layer 5 adjacent to section 3 b will contribute little to thebias field in the free layer 9 as its magnetization direction is mainlyalong the sensor edges and does not produce much magnetic charge on thestack edges, unlike the ideal case in FIG. 1. On the other hand, area 15where two differently oriented magnetizations meet in the HB layer 5,will have body charges that can produce a magnetic field to the sensorfree layer 9 as a biasing field. Thus, in reality, the hard bias fieldpresent in conventional TMR or CPP GMR read heads is theoretically notcomprised primarily of surface charges along the sensor edge, but isfrom body charges in the HB layer 5. The farther away the body charges(not shown) are from the sensor edge, the less amount of biasing fieldthat the charges can produce in the free layer 9, because the solidangle 50 from the charged areas relative to the free layer edges becomessmaller as distance between the body charges and free layer increases.

In FIG. 2 b, the concept of solid angle is explained in more detail andthe layers in read head 40 are removed to simplify the drawing. Thiscross-sectional view illustrates that a charged surface 51 in the HBlayer can be projected 54 onto a spherical surface 52 having an area “s”and ultimately focused to a point 53 on the free layer that is located adistance r from the spherical surface. From a top-down view (not shown),the spherical surface would appear as a circle, and from across-sectional view, the projection of the charged surface 51 ontopoint 53 would appear as a cone shape. The solid angle 50 is determinedby dividing s by r².

A reader shield-HB coupling effect can further degrade the HB strengthaccording to the scenario depicted in FIG. 2 a. In FIG. 3 a, an exampleis shown where there are no magnetic shields above or below the sensorstack. Two circled regions 17 a, 17 b are schematics of body chargedistribution in the HB layer 5. Exchange interaction between the HBgrains is also included because HB grains in read heads are usually notwell segregated by non-magnetic boundaries as they are in a magneticrecording medium. Therefore, the tilting of the magnetization away fromthe x-axis or sensor plane occurs farther away from the sensor edge andinsulating layer section 3 b. The magnetization direction in the HBlayer 5 is represented by arrows 18. Region 17 a represents a strongerbody charge and region 17 b has a weaker body charge. When top andbottom shields are added, especially when the top shield 16 (FIG. 3 b)follows the HB layer 5 and sensor stack topography, the HB layerstrength can be weakened by the shield-HB coupling of the HB and HBmagnetization rotation.

FIG. 3 b illustrates the effect of shield-HB coupling. The image of theHB in the top shield, for example, dashed arrows 19, has two effects.First the induced surface charge on the lower surface 16 a of the topshield 16 facing the HB layer 5 is opposite to the HB surface charge orbody charge that provides the hard bias field to the free layer 9 andthereby causes the effective bias field to the sensor stack to decrease.Secondly, the HB image (arrows 19) attracts HB magnetization 18 torotate towards the direction perpendicular to the bottom surface 16 a ofthe top shield to minimize Zeeman energy between the HB image and HBlayer 5. Additionally, with the top shield 16 conforming to the HB layer5 topography, the imaging effect is equally strong along the top surfaceof the HB layer which produces more canceling charges (not shown) on thebottom surface 16 a and also enhances the rotational behavior of themagnetization direction 18. Additional rotation of the HB magnetizationfrom shield-HB coupling will cause the body charges in the HB layer 5 tomigrate further inside the HB layer and away from the sensor stack edge.Note that region 17 a has weaker charges in FIG. 3 b than in FIG. 3 aand there is an additional region 17 c having weak charges formed agreater distance from section 3 b and the sensor structure. As a result,the effective solid angle of the HB body charge relative to the freelayer 9 becomes smaller and the HB field decreases correspondingly. Notethat region 17 b has a smaller solid angle 50 b than the solid angle 50a for region 17 a because of a larger distance “r” from the free layer9. Effective solid angle is related to charge density (bodyintegration×solid angle of the area divided by the M_(S) of the hardbias material.

The shield-HB coupling mechanism is more severe in narrow shield-shieldspacing examples. For high density magnetic recording beyond 1 Tb/in²,the areal density requires increasingly narrow down-track bit length andcross-track track width. To successfully read back narrower bit lengths,the read head's down track resolution must be improved which is usuallyachieved by narrowing the reader shield-shield spacing. In FIG. 4, anexample of narrow shield-shield spacing is shown. To narrow theshield-shield spacing between bottom shield 1 and top shield 16, a verythin capping layer 11 and a thin HB layer 5 are generally employed. Whena thin capping layer 11 is used, the distance between the HB layer 5 andbottom surface 16 a is reduced thereby leading to enhanced imaging ofthe HB layer by the top shield 16 and a greater shield-HB couplingeffect. In a scheme with a thinner HB layer 5, the HB cross section areadecreases proportionally with the HB thickness. As a result, the bodycharge that contributes to the bias field is reduced. Therefore, anarrower shield-shield spacing in a conventional read head causesdegradation of the HB field and a loss in device performance because ofthe combined effect of thinner capping and HB layers.

The shield-HB coupling induced HB field weakening can be mitigated byflat top shield 16 topography. However, flat topography normallyrequires chemical mechanical polishing (CMP) of the sensor stack. Assensor shield-shield spacing shrinks to several tens of nanometers inadvanced technologies, it is both technically and economically difficultto control CMP of the sensor stack with high fabrication yield. Forinstance, non-uniformities in the CMP process can easily cause largethickness variations in the capping layer 10 from one sensor to thenext. An alternative HB scheme is needed that avoids CMP and can providea robust and stable biasing field to the free layer in the sensor stackeven in narrow shield-shield spacing configurations.

During a search of the prior art, the following references werediscovered. U.S. Patent Application 2006/0132989 teaches longitudinalin-plane biasing. U.S. Patent Application 2005/0237677 describes a Cobased hard bias layer formed on an underlayer made of Ru, Ti, Zr, Hf,Zn, or an alloy thereof. In this CIP design, however, uncompensated backside charges will degrade the HB field generated by charges inside theHB layer.

U.S. Pat. No. 7,061,731 discloses an oblique deposition of a hard biaslayer in a direction normal to the preferred direction of anisotropy. Aseed layer is optional. U.S. Pat. No. 6,858,320 teaches that a seedlayer may degrade the orientation of the underlayer so a non-magneticintermediate layer is preferred. U.S. Patent Application 2006/0114622describes a hard bias layer formed on a seed layer on either side of asensor. An AP pinned structure on the hard bias layer reduces dependenceon the seed layer and increases coercivity. U.S. Patent Application2006/0087772 shows a hard bias layer formed on a CrMo seed layer. U.S.Pat. No. 7,072,156 discloses a decoupling layer between two hard biaslayers that acts as a seed layer to cause grains to have easymagnetization parallel to the interface between the layers. U.S. PatentApplication 2005/0066514 shows a hard bias seed layer made of Si and Cror CrMb.

U.S. Pat. No. 6,185,081 describes a seed layer that promotes in-planec-axis growth. U.S. Pat. No. 6,144,534 discloses a seed layer thatdisconnects the coherent crystal growth of the c-axis toward theperpendicular. U.S. Patent Publication 2005/0164039 teaches that thec-axis should be in-plane and not perpendicular.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a hard bias layerthat achieves a high concentration of surface charges along thesidewalls of the sensor stack in order to maximize the biasing field tothe free layer in the sensor stack.

A second objective of the present invention is to provide a hard bias(HB) layer for biasing a free layer in a magnetoresistive sensor thathas high HB coercivity and stability with small HB field variations.

A third objective of the present invention is to provide a hard biasscheme that utilizes a shield-HB coupling effect to increase HB layerstability and HB field strength.

A fourth objective of the present invention is to provide a hard biasstructure and a top shield that conforms to the HB layer and sensorstack topography to further enhance the shield-HB coupling.

A fifth objective of the present invention is to provide a hard biaslayer and junction configuration that generates a strong bias field fornarrow shield-shield spacing schemes in read heads for high densitymagnetic recording.

A further objective is to provide a method of forming a hard bias layerhaving an easy axis perpendicular growth that provides a robust, stable,and uniform hard bias field to longitudinally bias an adjacent freelayer in a magnetoresistive sensor.

According to the present invention, these objectives are achieved byproviding a bottom shield and a patterned sensor stack with oppositesides (sidewalls) formed thereon. An insulating layer is formed on thesidewalls of the sensor stack and on the bottom shield adjacent to thesensor stack. Above the insulating layer is a seed layer. A key featureis growth of the HB material on the seed layer such that the easy axisof the HB layer is oriented perpendicular to the seed layer. FollowingHB initialization, HB magnetization near the sensor will be along theeasy axis and perpendicular to the nearby sidewall of the sensor stack,resulting in surface charges as close as possible to the free layer.Body charges in regions of the HB layer along horizontal sections ofseed layer are not significant and only charges from grains along thesloped sensor stack edges are major contributors to the hard bias field.Surface charges near the top surface of the HB layer that could weakenthe HB field are effectively counterbalanced by induced charges from thetop shield because of strong shield-HB coupling. This HB stabilizationscheme may be fully utilized and HB field variations can be minimized byvarious embodiments that involve different magnetoresistive (MR)junction shapes and modified HB layer structures.

In one embodiment, a stack formed by sequential deposition of aninsulating layer, seed layer, and HB layer having an easy axis orientedperpendicular to the seed layer is disposed on the bottom shield andalong the sidewalls on opposite sides of the sensor stack. Thus, the HBlayer is comprised of a flat region above the bottom shield and twosloped regions near the sidewalls on opposite sides of the sensor stack.Above the HB layer and sensor stack is a capping layer that essentiallyconforms to the topography of the HB layer and top surface of the sensorstack. The top shield is formed on the capping layer and also followsthe topography of the HB layer. After HB initialization, the oppositepolarity surface charges on opposing sides of the sensor stack create abias field within the sensor. The top shield strongly couples with theHB layer to substantially compensate for charges at the top of the HBlayer and thereby maximize the effect of the bias field generated by thesurface charges on opposite sides of the sensor stack.

In a second embodiment, the flat region of the HB layer is removed toleave a HB region on the seed layer along either sidewall of the sensorstack. This configuration affords less HB field variation and reducesside reading at high track density because of additional side shieldingby the top shield.

Two additional embodiments involve extending the sensor stack slope intothe bottom shield while maintaining the same hard bias configurationsdescribed in the first two embodiments. These alternatives increase thevolume of the sloped HB region which in turn increases the number ofsurface charges along the sensor stack while improving the sideshielding from the top shield.

In a second series of embodiments, the capping layer between the HBlayer and top shield is removed so that the coupling between the topshield and HB layer is through direct exchange coupling of theirrespective magnetizations and not from a magneto-static field effectthrough a capping layer.

A third set of embodiments is similar to the first series of fourembodiments except that a soft magnetic layer and a non-magnetic spacerlayer are sequentially formed on the capping layer such that the softmagnetic layer may replace an equal volume of the top shield. The softmagnetic layer has a similar magnetic property as the top shield and isexchange decoupled from the top shield by the spacer layer.

In the fourth series of embodiments, the capping layer in the thirdseries of embodiments is removed such that the soft magnetic layer isformed on the hard bias layer and top surface of the sensor stack. Inthis case, the soft magnetic layer is direct exchanged coupled with theHB layer to compensate for charges on the HB surface.

In one aspect, the seed layer has an amorphous top surface that promotesa natural growth of the hard bias layer's perpendicular easy axis asdetermined by the hard bias layer's own crystalline properties.Optionally, the top surface of the seed layer may have a crystallinetexture that promotes an epitaxy growth of the hard bias layer'sperpendicular easy axis through lattice matching.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a magnetic read head having afree layer in a sensor stack that is stabilized by a conventionallongitudinal hard bias scheme.

FIG. 2 a is an enlarged view of the left half of FIG. 1 that illustratesthe hard bias (HB) easy axis orientation which is promoted by the seedlayer in a conventional read head.

FIG. 2 b is a cross-sectional view of the solid angle depicted in FIG. 2a that shows the projection of a charged surface onto a sphericalsurface.

FIG. 3 a is a cross-sectional view of the read head in FIG. 1 that showsbody charge distribution in the HB layer and FIG. 3 b depicts how thetop shield-HB layer coupling tilts the magnetization away from thesensor stack edge to degrade the HB field strength.

FIG. 4 is a cross-sectional view of a conventional read head with narrowshield-shield spacing and a thin HB layer that causes furtherdegradation of the HB field compared with FIG. 3 b.

FIG. 5 is a cross-sectional view of a read head in which the HB layerwith grains having an easy axis grown perpendicular to the seed layeraccording to the present invention.

FIG. 6 a is a cross-sectional view of the structure in FIG. 5 with theshields removed to show magnetization direction and surface charges inthe HB layer and FIG. 6 b shows how shield-HB coupling balances the HBtop surface charges in FIG. 6 a to enhance the HB field from surfacecharges formed along the sensor stack edge.

FIGS. 7 a-7 b are cross sectional views of one side of a sensor stack ina read head showing two different HB layer structures and junctionshapes according to the present invention.

FIGS. 8 and 9 are cross sectional views of the entire sensor stack in aread head and show magnetization directions in the HB layer according totwo embodiments of the present invention where a capping layer is formedbetween the HB layer and top shield.

FIG. 10 is an embodiment of the present invention wherein the sidewallsof the sensor stack in FIG. 8 are extended into the bottom shield.

FIG. 11 is an embodiment of the present invention wherein the sidewallsof the sensor stack in FIG. 9 are extended into the bottom shield.

FIG. 12 is an embodiment of the present invention wherein the cappinglayer between the HB layer and top shield in FIG. 8 is removed toprovide direct contact between the HB layer and top shield.

FIG. 13 is an embodiment wherein the capping layer between the HB layerand top shield in FIG. 9 is removed to provide direct HB layer—topshield contact.

FIG. 14 is an embodiment of the present invention wherein the cappinglayer between the HB layer and top shield in FIG. 10 is removed.

FIG. 15 is an embodiment of the present invention wherein the cappinglayer between the HB layer and top shield in FIG. 11 is removed.

FIG. 16 is an embodiment wherein a soft magnetic layer and an overlyingnon-magnetic spacer are inserted between the capping layer and topshield in FIG. 8.

FIG. 17 is an embodiment wherein a soft magnetic layer and an overlyingnon-magnetic spacer are inserted between the capping layer and topshield in FIG. 9.

FIG. 18 is an embodiment wherein a soft magnetic layer and an overlyingnon-magnetic spacer are inserted between the capping layer and topshield in FIG. 10.

FIG. 19 is an embodiment wherein a soft magnetic layer and an overlyingnon-magnetic spacer are inserted between the capping layer and topshield in FIG. 11.

FIG. 20 is an embodiment of the present invention wherein the cappinglayer in FIG. 16 is removed to give direct contact between the HB layerand soft magnetic layer.

FIG. 21 is an embodiment of the present invention wherein the cappinglayer in FIG. 17 is removed to give direct contact between the HB layerand soft magnetic layer.

FIG. 22 is an embodiment of the present invention wherein the cappinglayer in FIG. 18 is removed to give direct contact between the HB layerand soft magnetic layer.

FIG. 23 is an embodiment of the present invention wherein the cappinglayer in FIG. 19 is removed to give direct contact between the HB layerand soft magnetic layer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improved hard bias structure in a magneticread head and a method for forming the same that improves the HB fieldstrength, uniformity, and stability for longitudinal biasing of a freelayer in an adjacent sensor stack and is especially appropriate fornarrow shield-shield spacing configurations where conventional HBstructures lack HB field strength and control. Although the exemplaryembodiments depict a top spin valve in the sensor device, those skilledin the art will appreciate that the present invention may also apply tobottom spin valves or multilayer spin valves in sensor designs based ona GMR or TMR effect. Furthermore, the present invention encompasses asense current path that may be either current perpendicular to plane(CPP) or current in plane (CIP). The drawings are provided by way ofexample and are not intended to limit the scope of the invention. Thepresent invention is also a method of forming a HB structure in a readhead in which the HB growth has an easy axis perpendicular to theunderlying seed layer.

The concept of perpendicular medium development has already beendemonstrated in examples involving a magnetic recording medium where theeasy axis of the hard magnetic material is grown perpendicular to anunderlying layer with an easy axis tilting angle distribution of a fewdegrees as described in the following references: M. Zheng et al,“Seedlayer and Preheating Effects on Crystallography and RecordingPerformance of CoCrPtB Perpendicular Media”, IEEE Trans. Magn., Vol. 38,p. 1979 (2002); R. Mukai et al, “Microstructure Improvement of Thin RuUnderlayer for CoCrPt—SiO₂ Granular Perpendicular Media”, IEEE Trans.Magn., Vol. 41, p. 3169 (2005); and D. Vokoun et al, “Effects ofTb/Pt/Ru underlayer on microstructure and magnetic properties ofCoPtCr—SiO₂ perpendicular media”, J. App. Phys., Vol 99, p. 08E703,April, 2006. Although Publication No. US 2005/0237677 mentions aperpendicular HB growth, no shield-HB coupling is included and theteaching is mainly for a CIP head.

Referring to FIG. 5, a portion of a sensor stack in a read headstructure 30 is shown as viewed from an air bearing surface (ABS) plane.A bottom shield 1 made of permalloy, for example, is formed on asubstrate (not shown) that is typically ceramic and a patterned sensorstack is formed on the bottom shield by a well known method. Accordingto the exemplary embodiment of the present invention, the bottom layer 6in the sensor stack may be comprised of a composite such as a seed layerand an overlying AFM layer. Above the bottom layer 6 is sequentiallyformed a reference layer 7, a non-magnetic spacer or tunnel barrier 8, afree layer 9, and a capping layer 10. The compositions of the layerswithin the sensor stack are not described because the present inventionencompasses all sensor stacks and materials used to fabricate layerstherein. The patterned sensor stack has a top surface 10 a and asidewall 20 along one side. A second sidewall 21 along the opposite sideis illustrated in FIG. 8 and in subsequent drawings.

The insulating layer 3 was described previously and has sections 3 a and3 b formed on the bottom shield 1 and sidewalls 20, respectively. Thetop shield 2 above the capping layer 11 may be comprised of the samematerial in top shield 16 described previously but is generally moreconformal to the sidewalls of the sensor stack. There is a seed layer 22with a thickness between about 50 and 200 Angstroms disposed on theinsulating layer 3. The seed layer 22 may be comprised of Ru, Ta, Ti,TiW, TiCr, Cr, CrV, CrMo, CrW, and Al or any multilayered structure ofthe aforementioned compositions but is not limited to those examples. Akey feature is that the seed layer 22 promotes a hard bias easy axisgrowth perpendicular to the seed layer to give a hard bias (HB) layer25. This growth may be accomplished by two different mechanisms. In oneaspect, the seed layer 22 is substantially amorphous, especially at itstop surface (not shown) where the HB layer contacts the seed layer. Theamorphous phase at the top surface promotes a natural growth of the HBlayer's perpendicular easy axis (c-axis) determined by the crystallineproperties of the HB layer 26. Alternatively, the top surface of theseed layer 22 may be crystalline in order to promote an epitaxy growthof the HB layer's perpendicular easy axis through lattice matching. Thecrystalline top surface of the seed layer may derive its texture from anatural crystalline growth on an amorphous layer or by an epitaxy growthon a crystalline film. In one aspect, both the top surface of the seedlayer 22 and hard bias layer 25 may have a body centered cubic (bcc)lattice.

The HB layer 25 preferably has a thickness in the range of about 100 to300 Angstroms and may be comprised of CoCrPt or CoCrPtX where X may beB, O or other elements that can assist a perpendicular growth of the HBeasy axis. Optionally, the HB layer may be made of TbFeCo and ispreferably formed on an Al seed layer 22. Only certain combinations ofseed layer and HB layer will result in perpendicular easy axis growthsuch as the TbFeCo/Al combination or a CoCrPt HB layer on a Ru or TiWseed layer, for example. Another example is a [Co/Pt/Co]_(n) or[Co/Pd/CO]_(n) HB layer grown on a Pt or Pd seed layer where n is aninteger≧1 and can be adjusted according to the moment requirement. Theaforementioned HB layer/seed layer configurations can be grown by eitherphysical vapor deposition (PVD) or ion beam deposition (IBD) methods.

The inventor has discovered that temperature and other depositionconditions are critical for easy axis HB growth perpendicular to theseed layer 22. One example where a CoCrPt HB layer 25 is deposited on aTi₉₀W₁₀ or Ti seed layer 22 involves growing both layers in a IBD systemwith a Ti or Ti₉₀W₁₀ layer having a thickness of about 70 Angstroms orgreater. Furthermore, in order to achieve a uniformly thick HB layer 25on a seed layer 22 with topography as depicted in the preferredembodiments (FIGS. 8-23), it may be necessary to perform the seed layerdeposition and HB deposition in more than one step. For example, a firststep may involve a high deposition angle while a second step employs alow deposition angle. The deposition processes of the insulating layer,seed layer, and HB layer are generally performed with a photoresist mask(not shown) on the top of the patterned sensor stack. To minimize theoverspray, a shaper may be used in IBD systems. Typically, thephotoresist mask is removed after the HB layer 25 is formed. In someembodiments, portions of the HB layer 25 are removed by an etch processsuch as reactive ion etch (RIE) so that only hard bias regions along thesidewalls 20, 21 remain. To smooth the topography so that the HB layer25 and seed layer 22 are about coplanar with the top surface 10 a of thesensor stack, a mild CMP process may be employed. As mentionedpreviously, a HB layer 25 with perpendicular anisotropy may also beachieved by multilayer ferromagnetic/non-magnetic super-latticestructures such as [Co/Pt/Co]_(n) or [Co/Pd/Co]_(n).

Returning to FIG. 5, arrows 40 a represent easy axis of HB grains formedon the seed layer 22 above insulating layer 3 a and arrows 40 brepresent the easy axis of HB grains formed on the seed layer aboveinsulating layer 3 b. With the easy axes 40 a, 40 b orientedperpendicular to the underlying seed layer 22, initialization of the HBlayer 25 can be achieved similar to examples where the HB grains aregrown parallel to the underlying seed layer. For instance, a strongin-plane longitudinal field that overcomes the anisotropy field of theHB material may be applied along the x-axis direction (left to right)and aligns the HB magnetization in the same direction as the appliedfield. Once the field is withdrawn, the magnetizations of the HB grainsrelax to the uniaxial easy axis direction that has a smaller angle tothe longitudinal direction. HB initialization preferably occurs after atop shield 2 has been formed. For HB grains grown along the sidewall 20,the magnetization will be in the direction along the easy axis directionbut pointing downwards (toward the sidewall 20). Therefore, the charges(not shown) from the HB layer 25 are mainly surface charges from theseedge grains indicated by the circled area 23. This charge placement isactually the best situation for generating a strong HB field in thesensor stack and free layer 9 because the charges are at the closestposition to the sensor stack (sidewall 20) and the solid angle (notshown) from the charges is maximized.

For HB grains grown above insulating layer 3 a, their easy axis isperpendicular to the top surface 1 a of the bottom shield 1 as indicatedby arrows 40 a and the orientation of the magnetization is theoreticallyrandom after HB initialization. Charges will be formed near theinterface of HB layer 25 with seed layer 22 and capping layer 11 inregions above insulating layer 3 a. Random magnetization is not aconcern in these regions, however, since the solid angles formed bygrains therein relative to the free layer 9 are much smaller than thosegrown above insulating layer 3 b near sidewall 20. Therefore, thecharges in the HB layer 25 above insulating layer 3 a will be muchsmaller than the surface charges in area 23. A second considerationregarding random magnetization is that if the sensor stack andparticularly the free layer 9 is positioned near the center plane (notshown) of the HB layer 25, the field from the net charge, if any, on thetop and bottom surfaces of the HB layer above insulating layer 3 a willcancel each other as they are exactly the opposite sign and of the samemagnitude. Thirdly, with the top shield 2 present, the oppositelyoriented magnetizations in the HB layer 25 above insulating layer 3 acan easily form flux closure through the top shield-HB coupling and donot act on the free layer 9. As a result, only the charges from the HBgrains above insulating layer 3 b along sidewall 20 are majorcontributors to the HB field. It should be understood that the HB layer25 on the opposite side of the sensor stack has a magnetization alongsidewall 21 (FIG. 8) that is equal to the magnetization at sidewall 20but pointing in a direction perpendicular to sidewall 21.

It is reasonable to think that the HB regions where the two differenteasy axis growth patterns meet will be an area where an amorphous phasemay arise and cause variations in the HB field. However, this hard biasscheme does not depend on body charges to generate an HB field in thefree layer 9. Moreover, the HB layer 25 thickness can be greatly reducedto minimize the effect of the amorphous phase in a region where arrows40 a and 40 b intersect.

Unlike conventional hard bias schemes where top shield-HB couplingdegrades the hard bias field, the present invention takes advantage oftop shield-HB coupling to counterbalance charges at the top surface ofHB layer 25 that arise from HB edge grains and thereby increase the hardbias field. Referring to FIG. 6 a, grain magnetization orientation 24 aand 24 b after HB initialization is shown for a hard bias scheme withouttop and bottom shields. Note that the magnetization in area 23 (FIG. 5)produces surface charges near the interface of HB layer 25 and seedlayer 22 above sidewall 20 in FIG. 6 a that are of opposite sign to thecharges at the top surface 25 a of the HB layer above insulating layer 3b. Although magnetization 24 a has a random direction, magnetization 24b is oriented toward the sensor sidewall 20 and gives rise to positivesurface charges near sidewall 20 and negative charges on HB top surface25 a above insulating layer 3 b. According to one embodiment of thepresent invention, a portion of the top surface 25 b is coplanar withthe sensor stack top surface 10 a and an end of insulating layer 3 b.

Referring to FIG. 6 b, the example in FIG. 6 a is modified by adding abottom shield 1, a capping layer 11 on the HB layer 25 and top surface10 a, and a top shield 2 on capping layer 11. Since the top shield 2 isvery close to the HB top surface 25 a, the negative charges at the topsurface 25 a can be effectively counterbalanced by induced charges (notshown) from the top shield. As understood by those skilled in the art,this effect is attributed to the physical principle that magnetic fluxmust be continuous across an interface such as that between top surface25 a and top shield 2. It is believed that the HB magnetization 24 brotates slightly to a direction perpendicular to the HB top surface 25 aabove insulating layer 3 b in order to minimize Zeeman energy betweenthe HB magnetization 24 b and the so called HB image 26 in the topshield 2.

Referring to FIG. 7 a, a further modification of the hard bias structurefrom FIG. 6 b is shown. To fully utilize the shield-HB coupling assistedHB stabilization that was mentioned previously, and to minimize HB fieldvariation related to the amorphous phase at the intersection of two easyaxis growth directions, the HB layer 25 thickness is kept relativelyuniform above seed layer 22. Because the HB film thickness is uniformabove the sidewall 20, top shield 2 will conform to HB layer 25topography and bottom surface 2 a will parallel the sidewall 20 slope ina region above insulating layer 3 b. With a strong shield-HB coupling,the HB magnetization 24 b is more stiffly pinned in a directionessentially perpendicular to sidewall 20 by the coupling force than inFIG. 6 b. The thickness of the HB layer 25 above insulating layer 3 a isalso thinner than in FIG. 6 b. As a result, the volume of the possibleHB amorphous phase is reduced in the region where magnetization 24 aintersects magnetization 24 b and thereby has a smaller effect onvariation of the HB field. Capping layer 11 is preferably thin, on theorder of 5 to 30 Angstroms to promote strong top shield-HB coupling thatcan effectively counterbalance the negative charge on the HB top surface25 a and produce a stiffly pinned magnetization 24 b. Although thedrawing depicts only the HB layer 25 on one side of the sensor, thestructure of the HB layer on the opposite side is structurally the sameas shown in FIG. 8. Flux or magnetization 24 b and 24 c on oppositesides of the sensor stack is nearly entirely conducted by the top shield2 and forms flux closure (not shown) through the soft material of thetop shield at a relatively large distance away from the junction siteswhere magnetizations 24 b, 24 c produce surface charges near thesidewalls 20 and 21, respectively.

Referring to FIG. 7 b, an additional modification of the HB structureshown in FIG. 7 a is illustrated. The portion of HB layer 25 aboveinsulating layer 3 a has been removed to leave a portion of HB layerabove insulating layer 3 b that has a magnetization 24 b. The modifiedHB layer 25 has a top surface 25 a that is essentially parallel tosidewall 20, a bottom surface 25 c on seed layer 22 above section 3 bthat is parallel to top surface 25 a, one side 25 b that is coplanarwith top surface 10 a of the sensor stack, and a second side 25 d onseed layer 22 above insulating layer 3 a that is parallel to the topsurface 1 a of the bottom shield 1. Removal of the HB portion havingrandom magnetization 24 a is believed to have two effects. First, lessHB field variation is expected because the volume of the possibleamorphous phase mentioned earlier is further reduced and the exchangeinteraction between magnetizations 24 a is eliminated. Secondly, the topshield 2 now covers a greater portion of the sensor stack above sidewall20 thereby decreasing side reading at high track density as appreciatedby those skilled in the art.

Various embodiments of the present invention that take advantage of therobust HB field at the sensor sidewall 20 produced by magnetization 24 bin FIGS. 7 a-7 b will now be described. In the preferred embodiments,the top shield 2 is shown to be conformal to the HB layer surfaces 25 a,25 b, and the top surface 25 a above insulating layers 3 b, 3 c is drawnparallel to the nearest sidewall 20 or 21. However, the actual HB layer25 shape and top shield topography may vary and still retain the fulladvantages of the hard bias field generated in the exemplaryembodiments. In all embodiments, the key feature is the perpendiculargrowth of the hard bias easy axis as described previously. Further,insulating layer 3 and seed layer 22 are shown as separate layers butmay be a single layer in cases where an insulating layer is employedthat also promotes easy axis perpendicular growth. All layers in theread head 30 may be a single layer or composite layer having two or morelayers as appreciated by those skilled in the art.

Referring to FIG. 8, an embodiment is shown of a read head 30 that has asensor stack with layers 6-10 and sidewalls 20, 21 as describedpreviously. HB layer 25 has a shape as described in FIG. 7 a and amagnetization 24 b along sidewall 20 and a magnetization 24 c alongsidewall 21. Capping layer 11 is preferably thin as stated earlier withrespect to FIGS. 7 a-7 b. HB magnetizations 24 a, 24 b (24 c) aregenerated as previously described by HB initialization. In this case,the applied field during HB initialization is applied in the (+) x-axisdirection. As a result, the HB magnetization 24 b is essentiallyperpendicular to and pointing towards sidewall 20 and HB magnetization24 c is essentially perpendicular to and pointing away from sidewall 21.The HB images 26, 27 in top shield 2 stiffly pin the HB magnetizations24 b, 24 c in their respective directions. Additionally, the topographyof the top shield 2 follows the shape of the HB surfaces 25 b and 25 aabove the insulating layer 3 b in order to optimize the top shield-HBcoupling that compensates for charges at the surface 25 a therebystabilizing the HB field generated by HB magnetizations 24 b, 24 c. Notethat the random magnetization 24 a has little effect on the HB field forreasons stated earlier. This embodiment offers an advantage overconventional HB designs when top shield-bottom shield spacing is reducedin order to achieve high density recording because a strong HB field ismaintained at the free layer 9 due to strong shield-HB coupling in theportion of the HB layer having magnetizations 24 b, 24 c.

Referring to FIG. 9, a second embodiment is shown that is similar to theHB scheme in FIG. 8 except the portion of the HB region 25 aboveinsulating layer 3 a is removed as described previously with respect toFIG. 7 b. In other words, the HB regions having magnetizations 24 a areremoved on both sides of the sensor stack by an etch process, forexample, before overlying layers are deposited. This embodiment isbelieved to have an advantage over the HB scheme in FIG. 8 since thereis less HB field variation due to a decrease in the volume of thepossible amorphous phase in the HB layer at the intersection ofmagnetizations 24 a and 24 b and from an elimination of exchangeinteractions between HB grains having vertically oriented magnetizations24 a.

In a third embodiment represented by FIG. 10, the hard bias scheme isthe same as described with respect to FIG. 8 except that the slopes ofthe sensor stack (sidewalls 20, 21) are extended a certain distance intothe bottom shield 1. In other words, the top surface 1 a of bottomshield 1 that is below insulating section 3 a is now a distance d belowthe top surface 1 b that contacts layer 6 in the sensor stack. In otherwords, the section of bottom shield 1 below the sensor stack has agreater thickness than adjacent sections of bottom shield. Sidewalls 20and 21 may be extended into bottom shield 1 during the same etchsequence that patterns the sensor stack (layers 6-10) and beforeinsulating layer 3 is deposited as appreciated by those skilled in theart. Extension of the sidewalls 20, 21 into bottom shield 1 results in alarger region of HB layer 25 with magnetization 24 b, 24 c nearsidewalls 20, 21, respectively, compared with the first embodiment whichincreases the HB field for biasing free layer 9. An additional advantageover the first embodiment is that the bottom surface 2 a of the topshield 2 is lowered by a distance d with respect to the sensor stack andoffers additional protection against side reading at high track density.

Referring to FIG. 11, a fourth embodiment is depicted that is the sameas the second embodiment except the sidewalls 20, 21 have been extendedinto the bottom shield 1 as described earlier with respect to FIG. 10.The same advantages mentioned above regarding larger HB field forbiasing the free layer 9 and greater protection against side readingapply here when comparing the fourth embodiment to the second embodimentbecause of etching a distance d into the bottom shield 1. In this case,even greater side reading protection is offered than in the thirdembodiment because the bottom surface 2 a of the top shield 2 is loweredwith respect to its position in FIG. 10 by a distance equivalent to thethickness of HB layer 25.

A second series of four embodiments represented by FIGS. 12-15 arerelated to the first series of four embodiments (FIGS. 8-11) since thevarious shapes and positions of the HB layer 25 remain the same as inthe first series of embodiments. However, the capping layer 11 has beenremoved in FIGS. 12-15 so that the top shield 2 and HB layer 25 are indirect contact. HB images 26, 27 are present in top shield 2 but are notshown in order to simplify the drawings. The top shield 2 in FIGS. 12-15follows the topography of the HB layer 25 such that the bottom surface 2a contacts the top surfaces 25 a, 25 b of the HB layer. Therefore, thetop shield-HB coupling no longer occurs through a magneto-static fieldbut through direct exchange coupling of the top shield magnetization(not shown) with the HB magnetizations 24 b, 24 c. The magnetizations 24b, 24 c are oriented perpendicular to the sidewalls 20, 21,respectively, as in previous embodiments. Direct exchange coupling inthe second series of embodiments is believed to afford a greater HBfield to bias the free layer 9 than magneto-static field coupling in thefirst series of four embodiments.

FIG. 12 represents a hard bias scheme where the HB layer has essentiallyequivalent thickness over insulating layer sections 3 a, 3 b while theHB surface 25 b is coplanar with top surface 10 a of the sensor stack,and the bottom shield 1 has a uniform thickness and a top surface 1 awhich is coplanar with the bottom surface 6 a of layer 6 in the sensorstack.

Referring to FIG. 13, the hard bias structure is the same as in FIG. 12except a portion of the HB layer 25 that has random magnetizations 24 aabove the insulating layer section 3 a has been removed. The advantagesof the HB structure in FIG. 13 relative to FIG. 12 are the same as thosementioned with regard to a comparison of the second embodiment to thefirst embodiment (FIGS. 8-9).

Referring to FIG. 14, the hard bias structure is the same structure asin FIG. 12 except that the sidewalls 20, 21 are extended a certaindistance into the bottom shield 1. Thus, the top surface 1 a of bottomshield 1 that is below insulating layer 3 a is now a distance d belowthe top surface 1 b that contacts layer 6 in the sensor stack. Extensionof the sidewalls 20, 21 into bottom shield 1 results in a larger regionof HB layer 25 with magnetization 24 b, 24 c near sidewalls 20, 21,respectively, compared with the embodiment in FIG. 12 and therebyincreases the HB field for biasing free layer 9. Additionally, thebottom surface 2 a of the top shield 2 is lowered by a distance d withrespect to sidewalls 20, 21 and relative to its position in FIG. 12which offers additional protection against side reading at high trackdensity.

The hard bias structure in FIG. 15 is the same as in FIG. 13 except thatthe sidewalls 20, 21 are extended into the bottom shield 1. The portionof HB layer 25 having magnetization 24 b, 24 c has a greater volume thanin FIG. 13 which results in a stronger HB field for biasing the freelayer 9. Moreover, the bottom surface 2 a of the top shield 2 is nowlower by a distance d with respect to sidewalls 20, 21 and its positionin FIG. 13 to provide additional protection against side reading.

A third series of four embodiments represented by FIGS. 16-19 arerelated to the first series of four embodiments (FIGS. 8-11) in that thevarious shapes and positions of the HB layer 25 remain the same as inthe first series of embodiments. However, a soft magnetic layer 28 andan overlying non-magnetic spacer layer 29 that conform to the topographyof capping layer 11 have been inserted between the capping layer and topshield 2 in FIGS. 16-19. Moreover, the HB images 26, 27 in FIGS. 8-11are not shown here but are located in the soft magnetic layer 28adjacent to the sloped sides 25 a of HB layer 25. The soft magneticlayer 28 may be made of any soft magnetic material such as Ni₂₀Fe₈₀,CoFe, CoFeNb, CoFeZr, or CoFeB that has similar magnetic properties tothe top shield 2 and in one aspect replaces an equivalent thickness ofabout 1000 to 5000 Angstroms of the top shield. Note that the bottomsurface 2 a of the top shield still follows the topography of the HBlayer 25 and remains parallel to the HB surfaces 25 a, 25 b. Thenon-magnetic spacer 29 may be made of a metallic oxide such as aluminumoxide or titanium oxide and preferably has a thickness of about 50 to200 Angstroms. Non-magnetic spacer 29 breaks the exchange interactionbetween top shield 2 and soft magnetic layer 28. As a result, softmagnetic layer-HB coupling replaces top shield-HB coupling and occursthrough a magneto-static field to pin the HB magnetizations 24 b, 24 cin a direction perpendicular to the sidewalls 20, 21, respectively.

Referring to FIG. 16, the hard bias structure shown is the same asdepicted in FIG. 8 except for insertion of the soft magnetic layer 28and non-magnetic spacer 29 as described above. Likewise, the hard biasscheme illustrated in FIG. 17 differs from FIG. 9 because of theinsertion of the soft magnetic layer 28 and non-magnetic spacer 29between capping layer 11 and top shield 2. The top shield thickness maybe decreased by an amount equivalent to the thickness of soft magneticlayer 28.

Referring to FIGS. 18 and 19, the hard bias structure in read head 30 isthe same as shown in FIG. 10 and FIG. 11, respectively, except for theinsertion of the soft magnetic layer 28 and overlying non-magneticspacer 29 between the capping layer 11 and top shield 2. As a result,the soft magnetic layer-HB coupling interaction is by means ofmagneto-static coupling through the capping layer. In FIG. 18, thebottom surface 28 a of the soft magnetic layer 28 and bottom surface 2 aare lowered by a distance d relative to the x-axis and their position inFIG. 16. Likewise, in FIG. 19, the bottom surface 28 a and bottomsurface 2 a are lowered by a distance d relative to their positions inFIG. 17. As a result, the soft magnetic layer 28 and top shield 2 affordgreater protection against side reading in FIGS. 18-19 than in FIGS.16-17.

A fourth series of four embodiments represented by FIGS. 20-23 arerelated to the second series of four embodiments (FIGS. 12-15) in thatthe various shapes and positions of the HB layer 25 remain the same asin the second series of embodiments. However, a soft magnetic layer 28and an overlying non-magnetic spacer layer 29 that conform to thetopography of HB layer 25 have been inserted between the HB layer andtop shield 2 in FIGS. 20-23. The soft magnetic layer 28 and non-magneticspacer 29 were previously described in the aforementioned embodiments.In addition, the HB images (not shown) are present in the soft magneticlayer 28 adjacent to sloped sides 25 a as in the third series ofembodiments. Note that the bottom surface 2 a of the top shield stillfollows the topography of the HB layer 25 and remains parallel to the HBsurfaces 25 a, 25 b. Non-magnetic spacer 29 breaks the exchangeinteraction between top shield 2 and soft magnetic layer 28. In thiscase, soft magnetic layer-HB coupling replaces top shield-HB couplingand occurs through direct exchange coupling to pin the HB magnetizations24 b, 24 c in a direction perpendicular to the sidewalls 20, 21,respectively.

Referring to FIG. 20, the hard bias structure shown is the same asdepicted in FIG. 12 except for insertion of the soft magnetic layer 28and non-magnetic spacer 29 as described above. Likewise, the hard biasscheme illustrated in FIG. 21 differs from FIG. 13 because of theinsertion of the soft magnetic layer 28 and non-magnetic spacer 29between HB layer 25 and top shield 2. The top shield thickness may bedecreased by an amount equivalent to the thickness of soft magneticlayer 28.

Referring to FIGS. 22 and 23, the hard bias structure in read head 30 isthe same as shown in FIG. 14 and FIG. 15, respectively, except thatregions of the HB layer 25 having magnetization 24 a have been removed.As a result, the hard bias field provided by the HB regions havingmagnetization 24 b is expected to be more stable than in FIGS. 14-15.

In summary, the HB embodiments disclosed herein provide severaladvantages over prior art. These advantages include the following: (1)optimum bias field at free layer; (2) higher coercivity; (3) greaterstability against external field; (4) shield-HB coupling and topographyto stabilize the bias field; (5) insensitivity to narrow shield-shieldspacing; and (6) side shielding to prevent side reading from adjacenttracks. Each of these points is reviewed in the following paragraphs.

With regard to optimum bias field at the free layer, the highest surfacecharge density along the sidewall of the sensor stack is the saturationmagnetization M_(S) of the HB material and is achieved by growing theeasy axis of the HB layer perpendicular to the seed layer as describedherein. Compensation of the charges at the top surface of the HB layerthrough shield-HB coupling enables the M_(S) near the sensor sidewallsto provide the theoretically highest bias field at the free layer.Moreover, the surface charges in the HB layer are adjacent to the sensorsidewalls which is the nearest HB location to the free layer edge.Therefore, the bias field gradient on the free layer edge relative tothe free layer center is high and thereby helps to reduce free layeredge transient switching because of thermal agitation. The surfacecharge density according to the present invention is not affected by theslope angle of the sensor sidewall. On the other hand, in theconventional design shown in FIG. 1, the maximum surface charge of aperfectly longitudinally aligned HB layer is M_(S) sinθ, which dependson the slope angle θ, of the sensor sidewall. However, for real readhead structures, the charges are mainly located within the HB layer atsubstantial distances from the sensor sidewall where the solid angle issmall and the effective HB field is lowered as a result. Thus, from abias field point of view, the HB layer with perpendicular easy axisgrowth to the seed layer has an obvious advantage over the conventionallongitudinal case.

On the topic of higher coercivity, the conventional in-plane growth ofthe HB layer easy axis enables the hard axis to be aligned very wellvertically due to epitaxy growth from the seed layer, but the in-planeeasy axis of HB grains is random in nature. For a randomly orientedin-plane easy axis, the effective anisotropy energy is {K₁}=K1 dividedby the square root of N as stated in the following references: Y. Imryand S. Ma, Phys. Rev. Lett., 35, 1399(1975); G. Herzer, “Grain sizedependence of coercivity and permeability in nanocrystallineferromagnets”, IEEE Trans. Magn. Vol. 26, p 1397 (1990); and R.O'Handley, “Modern Magnetic Materials: Principles and Applications”,John Wiley & Sons, 1999, Chapters 9 and 12. In the aforementionedequation, K is the intrinsic anisotropy energy of the grains and N isthe number of grains within the exchange range of the HB material. In aread head, HB grains are not well segregated by the non-magnetic grainboundaries which causes the exchange interaction between the grains tobe high and the number of randomly oriented grains within the twodimensional exchange length to be not trivial. Therefore, for aconventional longitudinal HB scheme, the effective anisotropy energycould be much lower than the intrinsic energy of the grains.Additionally, as the critical grains that give rise to the biasing fieldare expected to be mainly located at the regions where the two growthpatterns meet and where the amorphous phase is likely to occur, theanisotropy energy could be further down graded by the softness of theamorphous grains. Consequently, the HB field is expected to have evenlarger variations. To the contrary, the perpendicular easy axis growthaccording to the present invention minimizes the randomness of the easyaxis orientation to within several degrees using a controlled epitaxygrowth or super lattice growth as mentioned earlier. Thus, the effectiveanisotropy energy of the critical slope grains is the same as or closeto the intrinsic anisotropy energy. Furthemore, with well controlledgrowth of the grains adjacent to the sensor slope, the amorphous phaseis practically avoidable along the sensor slope and HB field variationis small. With the additional modifications of HB layer as describedwith respect to FIGS. 7 a, 7 b and incorporated in the embodimentsrepresented by FIGS. 8-23, HB variation can be further improved.

Stability of the HB field against external fields is related to the factthat in-plane random easy axis orientation has a tendency for HB grainmagnetization to be permanently altered by external fields. Due to easyaxis randomness, HB magnetization of a cluster of grains defined by theexchange volume can have multiple low anisotropy directions that caneasily result in the cluster being tilted by a small amplitude externalfield and not returning to its initial direction. This multiple easyaxis scenario is especially detrimental if the cluster is contributingto the HB field, for example, on the boundaries of two lattice patterns,which causes HB field loss and sensor noise to increase duringoperation. As for the HB perpendicular easy axis (c-axis) growth of thepresent invention, the c-axis can be aligned within a deviation ofseveral degrees. Therefore, even under a relatively high external field,but not one that is high enough to cause a magnetization switching, themagnetization of HB grains along the sensor sidewall will return to theoriginal direction because of a lack of multiple easy axis directions.Thus, the HB structure described herein has a much higher stabilityagainst external field perturbations than prior art. Moreover, withshield-HB coupling that assists the perpendicular orientation of the HBmagnetization near the sensor sidewalls, HB field stability againstexternal fields is further enhanced.

A summary of the shield-HB coupling effect to stabilize the HB field isas follows. In conventional longitudinal HB schemes, shield-HB couplingshould be as small as possible because such coupling will weaken the HBfield strength and lead to sensor noise and instability. Prior artmethods to achieve low coupling involve modifications such as increasingthe distance between the top shield and HB layer by inserting a thickcapping layer or by employing CMP to reduce shield topography andminimize HB imaging in the shield. Unfortunately, a greater distancebetween the top shield and HB layer is against the requirement ofnarrower shield-shield spacing in future high density magnetic recordingdevices. CMP is not only expensive and a yield detractor, but is alsodifficult to implement while fabricating small sensors with narrowshield-shield spacing. The HB structures of the present invention arecompatible with top shield topography and take advantage of strongshield-HB coupling to improve the HB field thereby avoiding difficultand costly procedures like CMP. Instead, relatively easy process stepsare employed that can be readily implemented in existing manufacturingschemes. For example, a thinning down of the HB capping layer and auniform growth of the HB layer along the sensor sidewall and on the flatregion above the bottom shield can be realized with currenttechnologies.

With regard to the effect of HB layer thickness on HB field stability,conventional longitudinal HB schemes depend on the body charge for HBfield which makes the HB field strength very sensitive to HB volume andmeans that HB layer thickness changes lead directly to HB fieldvariation. The new HB structure and junction configuration disclosedherein is intended to make the HB field substantially independent of HBfilm thickness. As long as a crystalline structure can be maintained inthe HB layer near the sensor sidewall and the shield-HB coupling or softmagnetic layer-HB coupling is sufficient to compensate the HB charges atthe top surface of the HB layer, it is believed that bias field strengthwill be constant since the surface charge near the sensor sidewall willnot vary. Strong shield-HB coupling or soft magnetic layer-HB couplingcan be realized with a very thin capping layer on the HB layer and withtop shield topography or soft magnetic layer topography closelyfollowing the HB layer shape. Maximum coupling strength takes place inembodiments where the top shield or soft magnetic layer contacts the HBlayer and direct exchange coupling replaces magneto-static couplingthrough a capping layer.

Side shielding of the sensor stack is an extra feature provided by theHB configurations disclosed herein and can be optimized in certainembodiments by extending the sensor sidewalls into the bottom shield.This benefit is not available in conventional HB schemes since it is aby-product of promoting improved shield-HB coupling by way of strongtopography in the top shield or in the soft magnetic layer when it isused to replace a portion of the top shield in the present invention.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

1. A hard bias structure for providing a longitudinal bias field to afree layer in a magnetoresistive (MR) sensor having first and secondsloped sidewalls and a top surface and formed on a substrate, said hardbias structure comprises a hard bias layer that has an easy axis growthperpendicular to an underlying seed layer formed along the slopedsidewalls wherein said hard bias layer has a top surface and comprises:(a) a hard bias region on a section of the seed layer along the firstsloped sidewall that has a magnetization essentially perpendicular toand directed towards the first sloped sidewall; and (b) a hard biasregion on a section of the seed layer along the second sloped sidewallthat has a magnetization essentially perpendicular to and directed awayfrom the second sloped sidewall.
 2. The hard bias structure of claim 1wherein the MR sensor is based on a giant magneto-resistive (GMR) ortunneling magneto-resistive (TMR) structure and a sense current forreading the sensor is provided by a current in-plane (CIP) or currentperpendicular-to-plane (CPP) configuration.
 3. The hard bias structureof claim 1 wherein said hard bias regions are comprised of hard magneticmaterial with crystalline anisotropy comprising CoCrPt, CoCrPtX, orTbFeCo where X is B, or O that can assist easy hard bias axis growthperpendicular to the seed layer.
 4. The hard bias structure of claim 1wherein the hard bias layer has a [Co/Pt/Co]_(n) multilayerconfiguration where n is an integer ≧1 and said hard bias layer is grownon a Pt seed layer, or wherein the hard bias layer has a [Co/Pd/Co]_(n)multilayer configuration where n is an integer ≧1 and said hard biaslayer is grown on a Pd seed layer.
 5. The hard bias structure of claim 1wherein the seed layer may be a single layer or multilayer structure andhas a top surface contacting the hard bias layer that is comprised ofeither an amorphous material or a material having a crystalline texture.6. The hard bias structure of claim 5 wherein the seed layer iscomprised of one or more of Ru, Ta, Ti, TiW, TiCr, Cr, CrV, CrMo, CrW,and Al.
 7. The hard bias structure of claim 2 further comprised of aninsulating layer formed on the substrate and on the two sensor sidewallsin order to avoid shorting of the sensor and wherein said seed layer isformed on the insulating layer.
 8. The hard bias structure of claim 7further comprised of a top shield formed on the top surfaces of saidhard bias layer and sensor that substantially conforms to the topographyof the hard bias layer and thereby exchange couples with the adjacenthard bias layer to compensate for surface charges at the top surface ofthe hard bias layer, and wherein said substrate is a bottom shield in aread head.
 9. The hard bias structure of claim 8 wherein the two sensorsidewalls are extended into the bottom shield such that the sensor isformed on a first section of the bottom shield that has a greaterthickness than other sections of the bottom shield which contact saidinsulating layer and do not contact said sidewalls.
 10. The hard biasstructure of claim 7 further comprised of a stack comprising a lowersoft magnetic layer, a middle non-magnetic spacer, and an upper topshield that is formed on the hard bias layer, sensor, and portions ofthe underlying seed layer, and substantially conforms to the topographyof the hard bias layer such that there is exchange coupling between thesoft magnetic layer and hard bias layer to compensate for surfacecharges at the top surface of the hard bias layer, and wherein saidsubstrate is a bottom shield in a read head.
 11. The hard bias structureof claim 10 wherein the two sensor sidewalls are extended into thebottom shield such that the sensor is formed on a first section of thebottom shield that has a greater thickness than other sections of thebottom shield which are adjacent to said insulating layer.
 12. The hardbias structure of claim 8 further comprised of a conformal capping layerformed between the top shield and top surfaces of the hard bias layerand sensor and between the top shield and portions of the seed layerwherein the capping layer has a certain thickness range to enablemagneto-static coupling between the top shield and hard bias layer andthereby compensate for charges at the top surface of the hard biaslayer.
 13. The hard bias structure of claim 9 further comprised of aconformal capping layer between the top shield and top surfaces of thehard bias layer and sensor and between the top shield and a portion ofthe seed layer wherein the capping layer has a certain thickness rangeto enable magneto-static coupling between the top shield and hard biaslayer and thereby compensate for charges at the top surface of the hardbias layer.
 14. The hard bias structure of claim 10 further comprised ofa conformal capping layer formed between the soft magnetic layer and topsurfaces of the hard bias layer and sensor and between the soft magneticlayer and portions of the seed layer wherein the capping layer has acertain thickness range to enable magneto-static coupling between thesoft magnetic layer and hard bias layer and thereby compensate forcharges at the top surface of the hard bias layer.
 15. The hard biasstructure of claim 11 further comprised of a conformal capping layerbetween the soft magnetic layer and the top surfaces of the hard biaslayer and sensor wherein the capping layer has a certain thickness rangeto enable magneto-static coupling between the soft magnetic layer andhard bias layer and thereby compensate for charges at the top surface ofthe hard bias layer.